The present invention relates generally to digital-to-analog converters, and more particularly to a digital-to-analog converter for video and graphics applications which does not require a separate pin for calibration or reference voltage.
Digital-to-analog converters (DACs) are well known. DACs accept a digital input as logical pulses on parallel input lines and output a continuous waveform which represents the input numbers. This conversion is generally accomplished by either a binary weighted-resistor network or a matched resistor ladder network. Other techniques for conversion are also known, and often depend on the technology used to fabricate the DAC (e.g., CMOS, NMOS, TTL, etc.). In any event, several currents are weighted according to the significance they represent and summed at the input of an operational amplifier.
An n-bit DAC provides a well defined analog output which linearly varies from zero to its fall scale value as a digital input varies from 0 to 2.sup.n -1. Applications for high speed DACs include use in optical disk drives, instrumentation, communications, process control systems, and video data processing. In video processing systems for example, an analog video signal may vary from 0.5 to 2 times a nominal 1 V peak to peak range at the video signal receiving device while a typical power supply voltage may vary by 10% of its nominal value. High speed DACs are often sensitive to both process and temperature variations. As a result, an input digital video signal may not be properly converted to the appropriate analog range necessary for high quality video display. Accordingly, high speed DAC's generally include a reference pin used to calibrate the DAC output so that the DAC maintains a constant peak-to-peak voltage amplitude within power supply variation over process and temperature variation ranges. However, such pins can increase manufacturing cost and system complexity.
Consequently, there exists a need for a DAC that can be calibrated without requiring a separate external reference signal, such as a band gap reference or separate external pin.